So far, a separate movement management control system has been used from one mobile phone network system to another. It is contemplated nowadays to render it possible to apply the IP in the inter-conversion among packet networks of respective systems. In addition, such a system that allows for inter-operability among different radio network mediums has been a desideratum.
It is thus contemplated to implement such system in accordance with a proxy mobile IP system now being standardized by IFTF (Internet Engineering Task Force) for a movement management control system such as a mobile phone network or the WiMAX (Worldwide Interoperability for Microwave Access) network.
FIG. 6 shows a schematic configuration example of the system according to the proxy mobile IP system. Referring to FIG. 6, the system is composed of an MN (Mobile Node) 5, a MAG (Mobile Access Gateway) (1) 6-1, a MAG (Mobile Access Gateway) (2) 6-2 and an LMA (Localized Mobility Anchor) 7. The MN 5 may have communication via a radio communication domain Radio1 and further connection to the LMA 7 over the MAG (1) 6-1. The MN 5 also may have communication via a radio communication domain Radio2, and further connection to the LMA 7 over the MAG (2) 6-2.
With the mobile IP system, a mobile terminal registers a care of address (CoA), dependent on the network of the movement destination, using a registration notification message. A home agent sends an IP packet, addressed to a home address of the mobile terminal, to a care of address.
However, in applying the mobile IP system to a mobile phone network, there is raised a problem that, since the mobile terminal needs to possess a mobile IP control function, it is not possible to perform the movement management of the mobile terminal not having the mobile IP control function. There is also raised a problem that, in a collocate care of address (CCoA) system of IPv4 (Internet Protocol version 4) or in the IPv6 (Internet Protocol version 6) system, the IP encapsulation communication is needed in the domain of the radio communication, thus wasting the radio resource in an amount corresponding to the IP header.
The proxy mobile IP system is now under study in order to meet a demand that, as the control protocol of the mobile IP system is used, no special function is required of the mobile terminal, such that IP encapsulation communication is unnecessary in the radio communication domain (FIG. 7). Specified systems therefor are disclosed in Non-Patent Document 1 indicated below.
FIG. 7 shows that, if the MAG 1 and the MAG 2 are synchronized with each other, it is possible to manage order control when the MAG 1 and the MAG 2 send registration requests to the LMA. As a result of the registration requests, an IP tunnel is formed between the MAG and the LMA, so that IP packets may be transmitted encapsulated.
The IP packet, transmitted encapsulated from the LMA, is decapsulated by the MAO and transmitted in this state to the mobile terminal MN. A packet from the mobile terminal MN gets to the LMA by a reverse procedure to that described above.
The proxy mobile IP system operates as follows: The MAG is placed intermediate between a home agent and a mobile terminal. When the MAG has detected that the mobile terminal has moved into an area of an access network under the MAG's control, the MAG performs the registration, on behalf of the mobile terminal, in a home agent, in accordance with the proxy mobile IP system. An IP address of the MAG is used at this time as the care of address.
An IP packet, addressed to the home address of the mobile terminal, is transmitted from the home agent to the MAG, using IP encapsulation communication established between the home agent and the MAG. This MAG takes out the IP packet, addressed to the mobile terminal, and transmits the so taken out IP packet to the mobile terminal, using the radio link communication means between the MAG and the mobile terminal.
In handover processing, performed when the mobile terminal has moved to an area of another new MAG, the new MAG detects movement of the mobile terminal and performs registration in the home agent in accordance with the proxy mobile IP system. Since this proxy mobile IP system is such a system in which different MAGs perform the registration on behalf of the mobile terminal, there exists a plurality of sources of transmission of the registration notification messages.
Because of differential transmission delays between the respective MAGs and the home agent, there are cases where the order of the registration notification messages from the old and new MAGs is reversed when the messages arrive at the home agent. In a known manner, there persists a problem that, when the registration notification message from the old MAG has arrived at a belated timing, the IP packet is not sent to the new MAG.
In the mobile IP system, the mobile terminal appends a sequential number to the registration notification message to avoid the problem of order reversal. However, in the proxy mobile IP system, in which the scheme of global sequential number management may not be implemented, there persists the problem of order reversal.
Several solutions to overcome the problem of order reversal of registration notification messages are under research by the NETLMN WG (Network-based Localized Mobility Management Working Group of IFTF). In Non-Patent Document 2, indicated below, there are proposed five concrete solutions:
According to the first solution, a plurality of MAGs are synchronized with one another, using a time server, and the timing information is appended to the respective registration notification messages. A home agent performs order decision based on the timing information entered in the registration notification messages. This solution suffers a problem that synchronization to high accuracy is necessary.
According to the second solution, it is detected whether or not a registration notification message from an old MAG arrives within a predetermined time as from end of processing of the registration notification message from a new MAG. Only the first request from the old MAG is discarded. This solution suffers a problem that it depends on timer re-send processing of the old MAG, so that the processing is protracted, and another problem that a registration notification message from a new MAG, if any, is prioritized.
According to the third solution, the problem of the second solution that it depends on the timer re-send processing of the old MAG is to be overcome. It is thus detected whether or not a registration notification message from an old MAG arrives within a predetermined time after processing a registration notification message from a new MAG. The home agent explicitly returns an error only upon detection for the first time. The old MAG performs the registration again if a mobile terminal attaches to it and is under its control. This third solution suffers a problem that, if the registration notification message has arrived from the new MAG, it is prioritized.
According to the fourth solution, a new MAG performs initial registration when it has established a radio link and performs processing again after it has succeeded in the initial registration. That is, the MAG sends the registration notification message twice. This fourth solution suffers a problem that excess registration packets are necessary.
According to the fifth solution, a home agent returns an error in response to an initial registration notification message from a new MAG. At the same time, the home agent alerts a correct sequence number to the new MAG, which new MAG again performs registration using a correct sequential number. This fifth solution suffers a problem that excess registration packets are necessary.    [Non-Patent Document 1] “Proxy Mobile IPv6 draft-ietf-netlmm-proxymip6-0.1 txt” (NETLMM WG, Jun. 19, 2007)    [Non-Patent Document 2] “[netlmn] Issue:Timestamp vs Sequence Numbwe based logic”